The Strange, Small World Of Viruses

Science & medicine

Invisible And Stealthy, These Scraps Of Nucleic Acid Trick Their Victims By Becoming Part Of Their Genetic Code.

December 15, 1991|By Delthia Ricks Of The Sentinel Staff

Imagine a world of creatures where some are spherical, others cylindrical, a few polyhedral but most are beady and bubbly and swathed in protein coats.

They exist in a veritable biological zoo whose inhabitants are invisible to the naked eye but profoundly felt when they invade the cellular sanctum where human genes reside.

Welcome to the viral domain, a world where mere snippets of genetic material can carry the threat of wiping out entire populations, or reside quietly in cells for decades and be passed like benign genes to offspring.

Scientists are debating whether viruses - usually nothing more than strands of nucleic acid, DNA or RNA, enclosed in protein - are living organisms or inert chemicals. Containing hardly more than the instructions needed for their own reproduction, viruses lack an independent metabolism and need to penetrate a specific kind of host cell to replicate.

While scientists know how some viruses cause disease, an understanding of other strains remains elusive. And because viruses have evolved into such diverse entities, they manifest themselves in a wide range of living things: humans, plants and animals.

Scientists are finding that it is the very structure of a virus that helps it infect, duplicate and pose a threat in a world where virtually all other creatures are larger and more complex.

''Think of them as being like the surface of the moon,'' said biologist Sharon Wilder of Purdue University, describing the peak-and-valley structure of rhinoviruses, the culprits responsible for the common cold.

Though they are among the most prevalent viruses, developing therapies against them has posed a formidable challenge. Rhinoviruses, Wilder explained, can quickly change the structure of their ''peaks,'' tricking scientists and thwarting attack by the immune system.

Once a virus changes its protein coat, the human immune system's antibodies no longer recognize it and are unable to attach to the reshaped terrain. That explains, Wilder said, why people catch colds over and over again.

Rhinoviruses, so tiny that it would take more than 600,000 of them to span the length of this sentence, are also among the least complicated.

''AIDS and hepatitis viruses are much more complex. They have lipid membranes, spikes and attachment and penetration mechanisms,'' Wilder said.

And complex viruses, it seems, tend to cause some of the most complicated diseases.

Molecular biologist Bergit Fleurent of the Dow Chemical Corp., where viral structures are studied as part of the company's drug research, said the ''knobby'' protein coat of the human immunodeficiency virus sets the stage for AIDS.

That protein, she said, has the ability to attach itself to and penetrate the membrane of T4 lymphocytes, white blood cells that help the body orchestrate an attack on invading organisms.

Just why the virus has a proclivity for such a crucial cell remains a mystery, Fleurent said.

But this preference shows how viruses maintain what scientists call ''niches in nature.'' A common cold virus's niche - the place where it replicates and causes illness - is in the cells of the upper respiratory tract.

Hepatitis viruses home in on human liver cells; polio viruses have a specificity for cells of the nervous system.

To better understand how specific a niche can be, scientists at the National Institutes of Health who have injected the human immunodeficiency virus into laboratory mice have found that it does not cause disease in the animals.

Even when that virus infects monkeys, a close human relative, AIDS is not the result.

''Viruses are very specific,'' Wilder said of parasite-host relationships. Not all cells provide the type of environment a virus needs to replicate and survive.

Daniel Mancini, a Winter Park internist, marvels at the speed with which viruses can mutate, changing their identity yet remaining highly infectious.

Influenza viruses, which have some of the fastest mutation rates in nature, have changed significantly since the 1930s, when scientists first began keeping records on them.

''That's the reason why they come out with the vaccines every year,'' Mancini explained. ''The actual flu virus - its capsule or envelope - changes from year to year.''

Viruses are classified into different groups, and as a whole they cause an estimated 80 percent of all illnesses.

Most viruses are DNA viruses - their genetic material is stored in strands of deoxyribonucleic acid, which they use to replicate in their hosts. Other viruses are encapsulated strands of RNA - ribonucleic acid.

But there are even further classifications. Rhinoviruses and polio viruses, for instance, cause vastly different types of diseases but both belong to one group, the picornaviruses - or tiny RNA viruses - because of their size and configuration, akin to a soccer ball, Wilder said.

The AIDS virus and certain leukemia viruses are retroviruses, which, as the name implies, reverse the order of DNA-to-RNA transcription of genetic information - seen in virtually all other organisms - by virtue of an enzyme that allows the virus's RNA to be copied into DNA during replication.

Although hundreds of viruses have been identified, thousands of others that have never caused a human disease remain unknown.

Of the viruses that cause the common cold, for example, scientists have only scratched the surface of possibilities.

''There have been approximately 100 of them characterized to date, and each time (one of them has) a mutation, there's a new one to characterize,'' Wilder said.